Brain and behavior differ between males and females across vertebrate species. The zebra finch song system is a particularly useful model for understanding mechanisms that regulate development of sex differences in neural structure and function for many reasons, including extremely large morphological differences between males and females in a relatively simple neural circuit in which brain regions have clearly identified functions. Recent sequencing of the zebra finch genome provides new access to molecular tools and resources. As in mammalian species, estradiol (E2) can induce some masculinization, but a variety of pieces of evidence indicate that additional molecules are critical to normal male development. I propose to test a new, unique hypothesis, that the steroid hormone E2 acts in concert with masculine levels of expression of one or more sex chromosome genes to regulate appropriate male development. This work will provide critical novel information, as data on interactions between E2 and other molecules regulating sexual differentiation are very limited across species. The experiments involve three genes that we determined exhibit increased expression in specific song control nuclei in developing males compared to females. Collectively, the studies will provide a cohesive body of information integrating hormonal and genetic factors regulating development of brain structure and a learned social behavior - vocal communication, the primary modality used by humans. Specifically, we will use combinations of molecular, cellular, anatomical and behavioral approaches to test hypotheses about the relationships among E2 and specific molecules in development of forebrain structure and function. The ideas, which are not mutually exclusive, include that: (1) E2 increases expression of secretory carrier membrane protein 1 (SCAMP1), tubulin specific chaperone A (TBCA) and/or tyrosine kinase B (TrkB, the high affinity receptor for brain derived neurotrophic factor - BDNF); (2) These genes and the BDNF ligand modulate masculinization, in part by increasing responsiveness of the developing brain to E2; and (3) E2 and the genes/proteins can have complementary effects, including that E2 increases availability of BDNF, SCAMP1 and TBCA are positioned to facilitate BDNF's release, and TBCA and TrkB are localized such that they can increase BDNF's ability to act.